1. Field of the Invention
This invention relates to systems for enabling a driver of a motor vehicle to be alerted to the crossing or close proximity of a traffic line marking traffic lanes of a highway.
2. General Description of the Prior Art
Many accidents result from motor vehicles straying from one lane of traffic to another. Such events occur for a variety of reasons--simple inadvertance, the driver going to sleep or being intoxicated, and from poor visibility resulting from fog or rain. Thus it would be desirable for motor vehicles, trucks, buses, and passenger cars to be equipped with means for alerting a driver to such an occurrence to enable the driver to take timely corrective action. To this end, it has been previously proposed that photoelectric means be employed to sense when a vehicle closely approaches a traffic line on a pavement of a highway, traffic lines being generally of lighter color than the pavement. Unfortunately, however, the requirements for a practical and workable system employing such means are not readily apparent, and it does not appear that they have been met by previously proposed systems known to the applicants. For example, there is the problem of adequately distinguishing between ambient light and a change in light due to the detection of a line. A previously proposed system provides for the use of two photosensors connected in an electrical bridge circuit, whereby with equal illumination to both of the sensors, the bridge would be balanced. The photosensors are mounted on the vehicle so that one of them responds to light from the pavement under the center of the vehicle and the other to light near the side of the vehicle, and thus the latter would provide an increased output when that side of the vehicle approached a highway line, while the other one would not, causing an unbalance in the bridge and a signal to be provided to the driver of the vehicle. One difficulty with this system is that it requires that the pavement be artificially illuminated (at significant cost in energy) even in the daytime to maintain a sufficiently constant ambient light state for proper operation of the system. To accomplish this feat is indeed questionable because the intensity of natural sunlight is much greater than that of artificial light, and for one to achieve a perfect balance between the two sources is highly doubted. A second problem with this type of system is that in order to sense traffic lines on both sides of a vehicle, such as proposed by the present invention, it appears that separate illuminating sources would be required, and there would be created problems of still increased energy cost and of balance between light supplied by one and the other, or else two separate systems would be required.
Still another difficulty with the previously proposed system is that it fails to take into account the difference in sensitivity of existing photodetectors at different levels of ambient light, which are bound to occur even where supplemental light is employed, making it difficult or next to impossible to determine a fixed signal threshold, at which point a signal should trigger an alarm.
A further difficulty with the previously proposed system is that it provides for a signal output only during the period that an increased light (from a detected stripe) is sensed, and this may be for only milliseconds, a period which is too short for assurance that the signal will be observed by the driver.
A still further difficulty is that it appears that the previously proposed system would suffer from both temperature and general electrical instability.